An apparatus of this type is disclosed in U.S. Pat. No. 4,919,756 (Sawdai). The doctor blade is mounted in a bladeholder secured to a shaft extending parallel to the tip of the doctor blade. Each shaft end extends through a bearing in a two-armed impact-angle-adjust lever having one arm end pivotable in a pillow block located in alignment with the tip of the doctor blade. Axially outside the impact-angle-adjust lever a tipping lever is fixed to the shaft. An actuating cylinder is provided for swinging the tipping lever and, consequently, tipping the bladeholder between an active first position, in which the blade tip contacts the cylindrical surface of a Yankee dryer, and an inactive second position, in which a worn blade may be replaced. A jackscrew is connected between another pillow block and the other arm end of the impact-angle-adjust lever for pivoting the doctor blade around its tip. By continually adjusting the angular position of the doctor blade it is possible to reduce deleterious effects of doctor blade wear on the creping process by maintaining a substantially constant impact angle, and/or to substantially minimize the deleterious effects on a physical property of the paper web, e.g. the machine-direction tensile strength of the web, which would otherwise be caused by doctor blade wear. The disclosed apparatus has means for automatically continually adjusting the angular position of the doctor blade, and these adjusting means comprise means for being programmed with an empirically derived functional relation between the desired amount of doctor blade rotation and time.
The disclosed adjustable doctor utilizes a lever system of low mechanical stability. To avoid vibrations, which have a deleterious effect on the doctor apparatus and on the creped tissue paper produced, it would have to be considerably sturdier. Further, in order to minimize the forming of grooves in the cylinder surface of the Yankee dryer the doctor should be oscillated axially. An oscillation of the doctor is not provided for in the disclosed doctor apparatus and it would also require the apparatus to be able to absorb lateral forces. A modification of the disclosed adjustable doctor apparatus to overcome the above drawbacks would result in a space-requiring apparatus. An additional drawback is that it can not readily be substituted for an existing conventional doctor, because the attachments for the mounting of the doctor to a frame member of the paper machine are different.